Today's time domain multiplexed access (TDMA) cellular radio systems use frequency division duplex (FDD) to provide simultaneous radio transmission channels between the mobile station and the base station system. In FDD, a pair of simplex, radio frequencies with a fixed and known frequency separation called the duplex distance is used to define a specific duplex carrier or channel in the system. The frequency used to convey traffic to the mobile station from a base station is called the downlink frequency, while the frequency used to carry traffic from the mobile station to the base station is called the uplink frequency. In 900 MHz “Global System for Mobile Communications” (GSM 900) networks, for example, the duplex distance separating the uplink/downlink frequencies of any given channel is 45 MHz. Thus, if fu(n) is the frequency of the carrier for channel “n” in the uplink spectrum, and fd(n) is the corresponding frequency in the downlink spectrum, then:fu(n)=f0+B*n (MHz), andfd(n)=fu(n)+45 (MHz).Where, f0 is the carrier frequency associated with the lower edge of the frequency band used for uplink frequencies, and B is the channel spacing. As an example, in GSM 900, f0 is 890 MHz and B is 0.2 MHz.
FIG. 1 is a spectrum diagram that illustrates the above scheme. The channel shown at 101 has an uplink frequency separated by 45 MHz from a downlink frequency. Uplink and downlink frequencies in FDD networks are generally allocated and licensed in frequency blocks where all allocated frequency blocks together comprise an entire band assigned for a given radio technology. For each uplink block of frequencies, there is a corresponding downlink block of frequencies. For example, FIG. 2 illustrates how the uplink and the downlink spectrum are divided into five, 5 MHz blocks for GSM 900. Uplink blocks are denoted as U1–U5 and downlink blocks are denoted as D1–D5, respectively. In most cases, there is more than one operator that has been granted a license to operate a service within the same band. One operator can also have a license to operate multiple block pairs. In the example of FIG. 2, one operator has a license to operate on the block pairs (U1, D1), shown at 201, and (U5, D5), shown at 202.
With the above scheme, the total radio spectrum available is divided equally between the downlink and the uplink. This division can be referred to as symmetric radio frequency allocation. This symmetric radio frequency allocation has been used for circuit-switched applications such as speech that, until now, have been the dominant service provided by wireless service providers. However, the situation is changing dramatically as the mobile Internet emerges. Unlike speech, Internet traffic is mostly asymmetric as the data volume in the downlink direction far exceeds the data volume in the uplink direction. For example, during a World Wide Web (WWW) session, the files sent on the downlink are typically several magnitudes larger than the requests sent on the uplink.
Existing packet radio systems based on TDMA address unequal traffic on the downlink and the uplink by assigning a mobile station more timeslots on the downlink than on the uplink. In many cases, the net result of this asymmetrical timeslot allocation is that downlink frequencies reach full capacity while uplink frequencies are greatly underused.